During loading and unloading of a nuclear reactor with the fuel assemblies forming the core of the latter, a conveying carrier is normally used for supporting and displacing at least one new assembly to be put in position in the core, or the used assembly removed from the latter and to be transferred to a storage well which is outside the reactor enclosure. The carrier moves during its displacement at a convenient depth underneath the level of the protection water, by rolling on rails or similar guiding means between a receiving or removal position of a new or used assembly, provided outside the enclosure, and a loading or unloading position of the assembly, arranged inside said enclosure and in which it is removed from or positioned on the carrier. Between these two positions, the carrier slides or rolls on its rails, usually under the effect of a chain adapted to exert a pushing effort or reversely a traction effort, on the carrier, said chain being preferably controlled in the vicinity of the receiving or removal position of the assemblies outside the enclosure. The chain has the property of winding on itself as the carrier is drawn toward the latter position, enabling it to occupy only a reduced space inside the storage well, and also having sufficient rigidity when unwound to assume the function of a jack stem or arm of the carrier for bringing the latter to its loading or unloading position inside the reactor enclosure.
In prior art mechanism of this type the conveying carrier generally includes several controls, which require accurate coordination and necessitate the use of numerous and specialized personnel, distributed in the successive handling stations. Problems result when, for some reason, there is jamming or blockage of these control means, the carrier then running the risk of not being able to be brought back to its receiving or removal position for the new or used elements outside the enclosure. In such a case, it becomes necessary to resort to unblocking means which are heavy and cumbersome and the operation of which is complex, for example by using a pole, possibly telecontrolled, and which, through the head of protective water, acts on a rocker adapted to pivotably disengage the teeth of a drive pinion from a control rack for displacement of the carrier, before it is in position to bring it back, for example by means of a cable connected to the carrier frame and winding itself at its opposite end on the drum of winch.
A conveying carrier for nuclear fuel assembly, adapted to be transferred in the reactor building or in an annex building, with a reciprocating motion inside a horizontal communication duct along the axis of which it moves on carrying rails, is known from French patent FR-A-2 234 637. The movement of the carrier is effected with the assistance of a pusher arm having an end rigidly connected to a double chain, which is in turn driven on control pinions. When an incident occurs, particularly when the carrier is immobilized inside the reactor building, it is necessary to bring it back outside of said building; to this effect, one uses a cable provided with a clevis retained by a shearing pin, so that, when traction is applied to the cable, the pin breaks, thereby allowing the clevis to be blocked inside the end of the pusher arm, in order to permit retraction of the latter and that of the carrier due to the cable moving back upwardly. However, in this arrangement the pusher arm always remains rigidly connected to the carrier; if the incident is due to the arm itself, for example when blocked in the communication duct, the device fails and the cable does not allow the carrier to be returned to the annex building.